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  <div class="section" id="else-clause-on-loop-without-a-break-statement">
<h1><code class="docutils literal notranslate"><span class="pre">else</span></code> clause on loop without a <code class="docutils literal notranslate"><span class="pre">break</span></code> statement<a class="headerlink" href="#else-clause-on-loop-without-a-break-statement" title="Permalink to this headline">¶</a></h1>
<p>The <code class="docutils literal notranslate"><span class="pre">else</span></code> clause of a loop is executed when the loop sequence is empty. When a loop specifies no <code class="docutils literal notranslate"><span class="pre">break</span></code> statement, the <code class="docutils literal notranslate"><span class="pre">else</span></code> clause will always execute, because the loop sequence will eventually always become empty. Sometimes this is the intended behavior, in which case you can ignore this error. But most times this is not the intended behavior, and you should therefore review the code in question.</p>
<div class="section" id="anti-pattern">
<h2>Anti-pattern<a class="headerlink" href="#anti-pattern" title="Permalink to this headline">¶</a></h2>
<p>The code below demonstrates some potential unintended behavior that can result when a loop contains an <code class="docutils literal notranslate"><span class="pre">else</span></code> statement yet never specifies a <code class="docutils literal notranslate"><span class="pre">break</span></code> statement. <code class="docutils literal notranslate"><span class="pre">contains_magic_number()</span></code> iterates through a list of numbers and compares each number to the magic number. If the magic number is found then the function prints <code class="docutils literal notranslate"><span class="pre">The</span> <span class="pre">list</span> <span class="pre">contains</span> <span class="pre">the</span> <span class="pre">magic</span> <span class="pre">number</span></code>. If it doesn’t then the function prints <code class="docutils literal notranslate"><span class="pre">This</span> <span class="pre">list</span> <span class="pre">does</span> <span class="pre">NOT</span> <span class="pre">contain</span> <span class="pre">the</span> <span class="pre">magic</span> <span class="pre">number</span></code>. When the code calls the function with a list of <code class="docutils literal notranslate"><span class="pre">range(10)</span></code> and a magic number of 5, you would expect the code to only print <code class="docutils literal notranslate"><span class="pre">The</span> <span class="pre">list</span> <span class="pre">contains</span> <span class="pre">the</span> <span class="pre">magic</span> <span class="pre">number</span></code>. However, the code also prints <code class="docutils literal notranslate"><span class="pre">This</span> <span class="pre">list</span> <span class="pre">does</span> <span class="pre">NOT</span> <span class="pre">contain</span> <span class="pre">the</span> <span class="pre">magic</span> <span class="pre">number</span></code>. This is because the <code class="docutils literal notranslate"><span class="pre">range(10)</span></code> list eventually becomes empty, which prompts Python to execute the <code class="docutils literal notranslate"><span class="pre">else</span></code> clause.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">contains_magic_number</span><span class="p">(</span><span class="n">numbers</span><span class="p">,</span> <span class="n">magic_number</span><span class="p">):</span>
    <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">numbers</span><span class="p">:</span>
        <span class="k">if</span> <span class="n">i</span> <span class="o">==</span> <span class="n">magic_number</span><span class="p">:</span>
            <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;This list contains the magic number&quot;</span><span class="p">)</span>
    <span class="k">else</span><span class="p">:</span>
        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;This list does NOT contain the magic number&quot;</span><span class="p">)</span>

<span class="n">contains_magic_number</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="mi">5</span><span class="p">)</span>
<span class="c1"># This list contains the magic number.</span>
<span class="c1"># This list does NOT contain the magic number.</span>
</pre></div>
</div>
</div>
<div class="section" id="best-practices">
<h2>Best practices<a class="headerlink" href="#best-practices" title="Permalink to this headline">¶</a></h2>
<div class="section" id="insert-a-break-statement-into-the-loop">
<h3>Insert a <code class="docutils literal notranslate"><span class="pre">break</span></code> statement into the loop<a class="headerlink" href="#insert-a-break-statement-into-the-loop" title="Permalink to this headline">¶</a></h3>
<p>If the <code class="docutils literal notranslate"><span class="pre">else</span></code> clause should not always execute at the end of a loop clause, then the code should add a <code class="docutils literal notranslate"><span class="pre">break</span></code> statement within the loop block.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">contains_magic_number</span><span class="p">(</span><span class="n">numbers</span><span class="p">,</span> <span class="n">magic_number</span><span class="p">):</span>
    <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">numbers</span><span class="p">:</span>
        <span class="k">if</span> <span class="n">i</span> <span class="o">==</span> <span class="n">magic_number</span><span class="p">:</span>
            <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;This list contains the magic number.&quot;</span><span class="p">)</span>
            <span class="c1"># added break statement here</span>
            <span class="k">break</span>
    <span class="k">else</span><span class="p">:</span>
        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;This list does NOT contain the magic number.&quot;</span><span class="p">)</span>

<span class="n">contains_magic_number</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="mi">5</span><span class="p">)</span>
<span class="c1"># This list contains the magic number.</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="references">
<h2>References<a class="headerlink" href="#references" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><p>PyLint - W0120, useless-else-on-loop</p></li>
<li><p><a class="reference external" href="https://docs.python.org/2/tutorial/controlflow.html#break-and-continue-statements-and-else-clauses-on-loops">Python Standard Library - else Clauses on Loops</a></p></li>
</ul>
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